Object permanence refers to the understanding that objects continue to exist when they are not perceived. Object permanence develops gradually in human infants over the first two years of life and is characterized by a succession of stages. Earlier stages are tested with visible displacements (i.e., the subject watches an object disappear behind one or more screens and must then move the appropriate screen to find the object). Later stages are tested with invisible displacements (i.e., the object is first placed in a container and then the container is moved behind one or more screens and the object dropped behind one). In order to solve invisible displacement tasks, the subject must mentally represent the transfer of the object from the container to a location behind a screen. Thus, successful performance on these tasks is viewed as the beginning of representational thought. The purpose of this study was to compare the representational abilities of New World monkeys (squirrel monkeys at NERPRC) Old World monkeys (rhesus monkeys at UMass), and great apes (orangutans at the National Zoo). Individuals of all three species were tested on visible and invisible displacement tasks. Squirrel monkeys were successful on visible displacements but could not solve invisible displacements. On the latter tasks, the monkeys showed preferences for particular screens. Rhesus monkeys solved the visible displacements, but performed primarily at chance level on the invisible displacements. Two of nine rhesus monkeys actually performed above chance on several of the invisible displacement tasks. The remaining subjects became fixated on search strategies such as investigating one specific hiding location. Orangutans readily solved the visible displacements, but their performance on the invisible displacements was highly variable. Three subjects solved all the invisible displacement tasks whereas four subjects solved some tasks but performed no better than chance on others. For these four subjects, their performance is best explained as a complex strategy relating to the experimenter's movements. Both displacement tasks (visible and invisible) differ from each other not only in terms of representation but also in terms of memory requirements. Invisible displacement tasks traditionally involve more movements and thus require the animal to remember longer sequences of action. To rule out memory requirements as an explanation for the differential performance of many of the subjects on visible and invisible tasks, the tasks were modified using a new apparatus. In the new testing situation, the number of movements for visible and invisible displacements was made identical. The performance of squirrel monkeys was not substantially altered by this new procedure. Rhesus monkey performance was high on visible displacements and remained variable on invisible displacements with 2 animals performing better than chance on some aspects of the task. Orangutans performed essentially the same as on the standard set of tasks with some animals clearly comprehending invisible displacements and others performing at no better than chance levels. It seems unlikely that memory requirements account for the differential performance seen on visible and invisible displacement tasks. Furthermore, there appears to be no clearcut difference in performance between rhesus monkeys and orangutans. Instead, within species variability appears to be as great as between species variability. Thus, the antecedents for representational thought may be present to differing degrees in both Old World